The invention relates to the improvements in the field of air pollution control, specifically for removal of particulate matter (PM), nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO) and other toxic air pollutants from the exhaust of internal combustion engines and more specifically from diesel engines. Particulate matter, such as soot, emitted from diesel engines is very small—in most cases smaller than 1 micrometer, comparable in size with bacteria and 100 times smaller than human hair. These particles are complex, consisting of a carbon core, adsorbed hydrocarbons from diesel fuel and engine oil, adsorbed sulfates and inorganic materials from the engine wear. As early as 1988, the International Agency for Research on Cancer concluded that diesel particulate is probably carcinogenic to humans. Due to the small size of the particles, particulate matter is easily inhaled deep into lungs. NOx emissions from diesel engines also pose a number of health concerns, and may convert to nitric acid in lungs. CO is well known to be a deadly poison.
Diesel engine calibration can be adjusted within the temperature-atmosphere combustion map into regions of higher PM or NOx emissions. A combustion environment that contributes to higher PM emissions will naturally result in lower NOx and vice-versa.
In response to air quality regulations, vehicle manufacturers install pollution control devices in internal combustion engine exhaust systems. Existing engine pollution control devices often employ a ceramic honeycomb monolith having a coating of a noble metal catalyst. These pollution control devices catalyze the reactions of carbon monoxide and unburned hydrocarbons with oxygen at temperature ranging from 500 to 800 degrees Fahrenheit. Other devices employ catalysts that also catalyze the reaction of oxides of nitrogen.
Such catalysts are unsuitable for soot-laden gases that are produced by the diesel engines. Firstly, catalytic devices are ineffective at destroying soot. Secondly, the soot and other particulates remain as deposits on the ceramic monolith, preventing and restricting gaseous constituents from reaching the catalytic material or otherwise deactivating or poisoning the catalyst, greatly reducing the efficiency of the catalyst. The sulfur that is found in diesel and gasoline fuels can poison or deactivate the catalyst. Moreover, such devices induce a substantial back-pressure on the engine which reduces engine efficiency.
One of the methods used to reduce soot emissions from engine exhaust passes the engine exhaust gas through a ceramic filter, which filter can be periodically be replaced or regenerated. Such ceramic filters are only 85% efficient, impose significant back pressure on the engine and are expensive. For example, the loss of engine efficiency from filter back pressure can add between $3,000 and $6,000 in annual cost for a single school bus.
There are filters that regenerate themselves by burning some engine fuel, periodically oxidizing the accumulated soot. This extra fuel not only becomes expensive but also raises the total carbon footprint of the engine with the emission of more CO2.
Particles of soot usually have some positive natural charge when they leave the combustion chamber of a diesel engine. The technology of removal of soot particles by conventional electrostatic precipitators with direct current high voltage power followed by vertical cyclonic separators is well known and documented (U.S. Pat. No. 4,478,613 and others), and also has been used extensively for many years in the carbon black production plants. However, such systems are often used in stationary installations such as factories.
Electrical force on a charged 0.1 micrometer particle can be more than 1 million times the gravitational force on that particle. Accordingly, electrostatic precipitators are particularly effective in removing particles less than 1 micrometer (1 micron) in size. Electrostatic precipitators offer high capture efficiency for sub-micrometer particulates by using electrical forces to remove suspended particles from a gas stream. For removing particles from a gas stream, a direct current (DC) high voltage is desired.
Typically, three steps are involved in an electrostatic precipitator: charging, collection and removal of the collected particles. A corona discharge electrically charges suspended particles, which are attracted to and collected on opposite charged electrodes. Removal of the particles from the electrodes can be done by shaking or washing the electrodes.
With dry electrostatic precipitators, collection chambers are prone to build-up of accumulated particles. This build-up acts as an insulation, which reduces overall system efficiency. Collecting plate electrodes of dry electrostatic precipitators are usually cleaned by shaking the dust off or if the accumulated material is oily or sticky, by washing them with hot water and detergent.
With wet electrostatic precipitators, a liquid, usually water, continually washes particle build-up from the collection surface during the precipitation process. For motor vehicles, carrying liquid for the wet electrostatic precipitator adds weight to the vehicle, reducing fuel economy.
An electrostatic precipitator is most efficiently cleaned when it is not in operation. Therefore, having an electrostatic precipitator which can go for long periods between cleaning is desired. Conventional electrostatic filters for smoke and oil mist removal usually have ceramic insulators that separate ionizing and collecting plates. The ceramic insulators are exposed to the same gas stream with its particulate load as the ionizing and collecting plates, and after time, the surface of the insulators become coated surface with particles or oil mist. For oil mist applications, this coating does not always impede the ability of the unit to operate, but since the soot particles from diesel engines are conductive, a coating of soot particles on the surface of the insulators will render the conventional unit inoperable.
In an electrostatic precipitator, corona power is the product of operating voltage and the operating corona current. Efficiency of an electrostatic precipitator is directly proportional to its corona power. Corona power is limited by internal sparking between ionizing and collecting electrodes. Each time a precipitator sparks, it reduces voltage and correspondingly, particulate collection efficiency. Corona current may also be suppressed when the incoming exhaust has an abnormally high load of soot, such as when the vehicle is accelerating. A large number of particles entering the precipitator may receive a charge, but not have had a chance to be collected on a collecting electrode. These charged particles prevent electron movement between the discharging and collecting electrodes and therefore manifest as less current available for charging and collection, also known as corona current suppression.
It is known in the art that an ionizing electrode functions most efficiently when the discharge points are as small as possible. Existing ionizing electrodes are often in the form of wires, but discharging electrodes have also been used in the form of a flat plate with points on a needle located along the outer periphery of the plate or rod shape of the ionizing electrodes. (See, H. Surati, M. Beltran and I. Raigorodsky (now known as Isaac Ray), “Tubular Electrostatic Precipitators of Two-Stage Design”, Environmental International, Vol. 6, pp. 239-244, 1981). Ionizing wires, such as have been used in the industry, are usually very thin (twice as thick as human hair), presenting substantial operating problems due to frequent breaking, and preventing their use in moving vehicles.